Certain magnetographic printing systems such as those described in the above-mentioned application utilize a transfer electrode to generate an electrostatic field for transferring successive lines of toned images from an imaged magnetic tape medium to paper. In such systems, there can be a tendency for portions of the image of a previously transferred (printed) line to become disturbed or lost (i.e., toner from portions of the image of the previously transferred line may become disturbed or lifted from the paper by the tape) resulting in degraded, non-uniformly toned (non-uniformly opaque) image lines. Generally this so-called "lift-off" (removal) of a portion of a previously transferred line occurs during transfer of a subsequent image line to paper, when a portion (generally, an edge) of the tape is brought into contact with the previously transferred line, and/or when the electrostatic field generated by the transfer electrode cause the transferred particles to migrate away from the previously transferred image and toward the source of the field (viz., the transfer electrode). This toner disturbance or removal problem may become aggravated due to formation of exposed charge-accumulating areas on the tape occasioned by, for example, tape wear.
What is needed and would be useful, therefore, is an apparatus which, after a line of toned images is transferred to the toner-receiving medium (e.g., paper), would be capable of preventing disturbance and/or loss of toner from the transferred image despite contact or engagement of the transferred image with a portion of the tape or other image-carrying medium, and despite the action of the transfer electrode during image transfer.
Thus, in accordance with the broader aspects of the illustrated preferred embodiment(s) of the present invention, apparatus is provided which is capable of substantially preventing disturbance and removal of toner particles from previously transferred images. The apparatus operates as part of a magnetographic printing system, wherein a magnetic tape having a toned image thereon is capable of being moved into contact with and thereafter away from a selected front surface of a paper, thereby transferring charged toner particles from imaged areas on the tape to the paper's front surface.
The apparatus comprises an electrode positioned proximate to a rear surface of the paper opposite to the transferred particles (the transferred particles representing a previously transferred line) on the front surface, the electrode being responsive to an applied signal having a predetermined polarity, for example opposite to that of the transferred particles, for creating an electric field restraining the transferred particles to the front surface, thereby preventing migration of the particles during the transfer operation, and preventing disturbance and removal (lift-off) of the previously transferred particles from the paper by the tape when the tape is brought into contact with and subsequently becomes separated from the paper during the current transfer operation. The position of the electrode relative to the transferred image and to the image to be transferred from the tape, as well as the voltage applied to the electrode, are selected such that the electric field created by the electrode is effective to restrain the transferred particles from leaving the paper yet does not adversely affect particles being transferred or particles situated on the tape to be transferred to paper.